1. Field of the Invention
The present invention relates to a Y-branch waveguide.
2. Description of the Related Art
Conventionally, a Y-branch waveguide has been used as a waveguide type element for recombining or branching light. A Y-branch waveguide is a basic element making up various waveguide devices, and is an essential element in the construction of optical network systems.
As an element having the function of causing branching of light, besides the Y-branch waveguide there is a directional coupler, such as that disclosed in Japanese Patent Laid-open No. Hei.3-172804 or Japanese Patent Laid-open No. Hei.5-119220. However, the propagation constant of a directional coupler varies considerably with the wavelength of an incoming beam, which means that the branching ratio depends of the wavelength and varies a lot. Accordingly, with a directional coupler the practicable wavelength band is limited to 5-10 nm. Also, in the case of a directional coupler, it is extremely difficult to make an interval between waveguides or a coupling length the size it was designed at, which means that there are very large variations in characteristics between products.
On the other hand, it is well known that the branching ration of Y-branch waveguide type elements is less wavelength dependent, which means that there is a practicable wavelength band of about 100 nm, for example, and also there is reduced variation in characteristics caused by dimensional errors in the manufacturing process such as with a direction coupler. For the above described reasons, current Y-branch waveguide are in general use.
A conventional Y-branch waveguide will now be described below with reference to FIG. 7. FIG. 7(A) is a drawing schematically showing the structure of a conventional waveguide.
As shown in FIG. 7, this Y-branch waveguide 101 comprises a single combining side waveguide 103, two branching waveguides 105a and 105b, and a junction area waveguide 107. The junction area waveguide 107 connects between the combining side waveguide 103 and each of the branching waveguides 105a and 105b. The junction area waveguide 107 is has a tapered shape where the width at the branching is wider than the width at the combining side.
The waveguide 101 shown in FIG. 7(A) takes incoming light from the combining side waveguide 104, and outputs light to the two branching waveguides 105a and 105b as two beams of branched light having power distributed at a specified ratio. This branching ratio can be set as desired, and is typically equally distributed.
As is also well known, in order to reduce light loss, theoretically a branching width H of a section sandwiched between the branching waveguides 105a and 105b connected to the junction area waveguide 107 is preferably zero.
However, as shown in FIG. 7, with the conventional Y-branch waveguide 101, it is well known that a rounded portion 109 is formed in part of the junction area waveguide 107 sandwiched between the two branching waveguides 103a and 103b. The rounded portion 109 has a portion that is to be cut out into a V-shaped slot that is originally U-shaped due to limitations of ultrafine machining techniques used in the manufacturing process, namely the limits of patterning and etching machining precision, which means that some of the waveguide material remains in the region that is cut out.
FIG. 7(B) is an enlarged view of part of FIG. 7(A), and shows the vicinity of where the branching waveguides 105a and 105b and the transition side waveguide 107 are connected.
As shown in FIG. 7(B), theoretically the branch width H is preferably zero, so the Y-branch waveguide is designed as a structure having a V-shaped valley section 109a as shown by the dotted line (in this case, the branch width =0), but as shown by the solid line, in actual fact a rounded portion 109 (that is, remaining parts of the wave guide material) occurs.
If a rounded portion occurs in this manner, the branch width H of the branching waveguides 105a and 105b becomes wider than the designed value, causing the effective length of the junction area waveguide 107 to also be longer than the designed value.
The length of the junction area waveguide 107 has essential parameters that determine the characteristics of the Y-branch waveguide, as disclosed I reference I (Japanese patent Laid-open No. Hei.5-11130). Accordingly, if the effective length of the junction area waveguide 107 varies, it is not possible to obtain the characteristics that were designed for.
One proposition to deal with this type of problem is, as disclosed in reference II (Japanese Patent Laid-open No. Hei. 3-2451070), a Y-branch waveguide in which the occurrence of a rounded portion is suppressed by previously ensuring or allowing for it at the time of designing the branch width H. With this Y-branch waveguide, since a rounded portion does not arise, it is possible to achieve the designed for characteristics.
For these reasons, currently Y-branch waveguides with a U-shape between two branching waveguides, as shown by the solid line in FIG. 7, are frequently used.
However, with this type of Y-branch waveguide, even if the designed for characteristics are obtained, there is considerable non-conformance in guided wave mode field distribution between the transition guide wave and the branching waveguides.
FIG. 8 is a drawing for describing a Y-branch waveguide having a specified branch width disclosed in reference II. FIG. 8 schematically shows phase fronts of propagated light in the case where light is injected from a merging side waveguide.
As shown in FIG. 8, with this Y-branched waveguide 201 an advancing direction k1 of the propagating light in the junction area waveguide 207 (a normal direction of the phase front) and an advancing direction k2 (an ideal advancing direction) of propagating light in the branching waveguides 205a and 205b are different. At this time, overlapping of field distributions for guided wave mode propagated light close to a boundary of the junction area waveguide 207 and the branching waveguides 205a and 205b becomes slight. As a result, propagated light not injected to the branching wave guides 205a and 205b becomes emission mode and is discharged to a substrate side, which means that light loss in the Y-branch waveguide is increased.
Accordingly, in the Y-branch waveguide having a branch width between branching wave guides it is preferable to have a Y-branch waveguide that can reduce loss due to mode field distribution non-conformance between the junction area waveguide and the branching waveguides.
The Y-branch waveguide of the present invention comprises a single merging side waveguide, two branching waveguides, and a junction area waveguide provided between the merging side waveguide and the branching waveguides and formed in a tapered shape being thicker at the branching than at the merging side, in which there are provided two straight waveguides parallel to each other, each being connected to the junction area waveguide and to a different single branching waveguide. These straight waveguides extend in the same direction as the direction of propagation of light inside the junction area waveguide, when light is injected from the merging side waveguide.
With this structure, since the junction area waveguide and the branching waveguides are connected through the straight waveguides, it is possible to obtain the following type of effects.
Specifically, first of all light injected externally, for example, from the merging side waveguide, becomes propagated light having a specific propagation mode. If this propagated light is injected into the junction area waveguide that is tapered widening in the advancing direction, it is propagated without causing variation in the advancing direction.
After that, propagated light is branched and injected into each of the straight waveguides. With this structure, the extending direction of the straight waveguides is aligned with the advancing direction of propagated light in the junction area waveguide, which means that loss between the junction area waveguide and the straight waveguides is extremely low.
In this way, light distributed in each straight waveguide propagates through the straight waveguide. At that time, since one waveguide system is constructed of the whole of the two straight waveguides, a phase accelerator effect arise in part of the straight waveguides. At this time, therefore, the distributed propagated light propagates through the straight waveguide while changing direction (a detailed description of the reason for this will be given later).
As described above, the direction of propagating light in the straight waveguides can be aligned with the direction of propagating light in the vicinity of the boundary of the straight waveguides and the branch waveguides. As a result, non-conformance of mode field distribution is mitigated at the boundary sections. Accordingly, with the Y-branch waveguide of this invention loss caused by mode field distribution non-conformance can be reduced.